Passive Top Balancing ? How/Why does it work ?

[Boondock Saint]

That's in a circuit Bob, where there is supposed to BE flow. You can't state voltage potential where there is no voltage, which is a measure of potential, which is between the positive and the negative. Therefore there is no potential, like me banging Kate Beckinsale. Just defies some law of physics.

Get your VM and go measure between the negatives of a row of not connected batteries. Do you see voltage? NOPE. There isn't any. Just because you connect a wire to them doesn't make them sprout and suddenly want to play ball. They are unaware of each other. You ever try to run a device on just a single lead of wire? Won't happen. You need both leads and the potential, the voltage between them.

Now you could use a wire or resistor between the two rails and KAPOW of course there's some potential there, that could fry your chick real good.


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[Bob B]

Ok .... You guys seem like you have gone off the deep end .... so @snoobler ..... I guess you are going to have to figure out how to explain this .... I gotta get up early tomorrow.

 

[2Big2B]

Can we get Will to mediate?

 

[Boondock Saint]

Huh? by what path?

The path of least resistance. haha get it . . the . . nvrmnd.

 

[snoobler]

2Big2B

Have you ever jump started a car?

 

[snoobler]

I'll repeat the question.

Have you ever jump started a car?

 

[Boondock Saint]

2Big2B

Have you ever jump started a car?

Touche. An example of a charged battery supplementing a depleted battery with electromotive force. In this example the lower charged battery draws from the higher charged battery, essentially drawing down the higher charged battery. The circuit here would be through the opposing battery electrolyte to the opposite pole.

If the good battery was fully charged at 12.5v and the low battery was down to 11v . . if you just left them connected and didn't try to start the car, eventually the low battery would come up and the full battery would come down, but they still never exactly meet in the middle. By what period of time would that ever happen?

In a LifeP04 rail with 32 cells, are you suggesting they will all top balance and all come to the same SOC over time in this passive common connectivity?

 

[Hedges]

If you connect two lead-acid batteries in parallel (one new & full charged, one on its last legs) they will balance without connecting a charger.
If you connect two capacitors in parallel, one 100 uF charged to 50V and one 100 uF charged to 100V, they will balance.

EE class time!
1) What is the final voltage of the capacitors?
2) How much energy was in each capacitor to begin with, and how much after they're balanced?
3) <extra credit> What happened to the missing energy??

If you connect two lithium cells in parallel and they are very far apart in SoC (so their voltages are significantly different, current will flow and some balancing will occur. However, the chemical reactions require some extra push, and charging/discharging the cells is not 100% efficient. When they are close in voltage, they apparently won't continue transferring current to the point of being well balanced.

When you parallel two cells, circuit is complete and current could flow, but it doesn't balance well. Besides, same voltage at a low SoC doesn't make them balanced. Pushing both cells in the same direction (both toward more charged or both toward less charged) and up onto the steep part of the SoC/voltage curve, is what gets them balanced.

(It's Scaramouche; you've got to get your literary characters correct.)

 

[toms]

I really thought i answered the question with the first paragraph of post #2.

Not sure which part was hard to understand.

 

[svetz]

...I have already top balanced my 8 cells both independently and in parallel with my bench power supply to 3.65v on all cells. I am about ready to hook up my BMS and move forward. Do I actually need to remove the power supply and just leave them all connected to each other in parallel overnight before proceeding?

Typically after charging the batteries will "settle" a little, so that's why some wait a bit. If all the cells have close to the same voltage, you're ready to go.

...If I try testing for current between the first negative terminal and the last in my string of 8 cells set up for parallel top balancing I seem to be seeing a slight voltage difference on my Fluke meter - consistently.

If you've just top balanced them and assembled the string, they should all have the same cell voltage. If you're seeing fluctuations in the 100ths of a volt, it's probably just meter sensitivity. If it's something else let us know, perhaps someone can say what's going on.

It is common, after a few cycles, for voltages of cells in a string to start to drift. That's because the cells have slightly different internal resistances and it's why we need a BMS.

People have corrected me who seem to think that just having a string of cells hooked up in parallel - not connected to anything else - will cause them to equalize. Are they correct?

It helps to think about this from a frame of reference. Electrons go in or out of a terminal depending on if you're charging or discharging. When a battery is discharging, electrons come from the negative terminal and traverse a circuit to the positive terminal.
When charging, electrons go into the negative terminal and come out the positive terminal.

Electrons move whenever there is a voltage potential that exceeds the resistance, you're probably familiar with this as Ohm's law, V=IR.

So, what happens when we hook two batteries up in parallel at different voltages? The resistance is very low, so electrons will flow from the higher voltage battery into the lower voltage battery until they are the same voltage. So, putting batteries in parallel will equalize the voltage.

Hope that helps!

 

[wholybee]

LiFePO4 introduces an issue that the voltage curve is very flat. That is separate from the question about whether one cell will discharge into another without any device like a charger. It will, and in fact is is simple to prove.

Current will flow from a terminal of higher potential, to a terminal of lower potential. With a single battery in a typical circuit, from the positive terminal, through the circuit, to the negative terminal. However, consider the following test:
Take 2 batteries, a 12V and a 6V.
Connect the negative terminals together. Do not connect the positive.
Use volt meter and measure from one positive to the other. You will measure 6V, the difference between the 12V and 6V terminal.
Connect a 6V light bulb between the two positive terminals. It will light up. Current is flowing from the 12V to the 6V battery, through the light bulb. You can measure the current with an ammeter.
If connected long enough, the 12V battery will deplete, and the 6V battery will overcharge.
Do not connect them directly without the light bulb, because the current will be very large, and dangerous-just like shorting the positive and negative together on a 6V battery,.

This is the same as paralleling 2 cells to balance them, except the voltages are much closer together. A cell charged to 3.3V and one charged to 3.4V will have 0.1V of difference between them. That is enough for some current flow, and enough for the cells to (in theory) balance. With Lead Acid it will mostly work. With LiFePo4 it will not, because the 2 cells will converge to 3.35V before the state of charge between them is the same.

 

[Bob B]

LiFePO4 introduces an issue that the voltage curve is very flat. That is separate from the question about whether one cell will discharge into another without any device like a charger. It will, and in fact is is simple to prove.

Current will flow from a terminal of higher potential, to a terminal of lower potential. With a single battery in a typical circuit, from the positive terminal, through the circuit, to the negative terminal. However, consider the following test:
Take 2 batteries, a 12V and a 6V.
Connect the negative terminals together. Do not connect the positive.
Use volt meter and measure from one positive to the other. You will measure 6V, the difference between the 12V and 6V terminal.
Connect a 6V light bulb between the two positive terminals. It will light up. Current is flowing from the 12V to the 6V battery, through the light bulb. You can measure the current with an ammeter.
If connected long enough, the 12V battery will deplete, and the 6V battery will overcharge.
Do not connect them directly without the light bulb, because the current will be very large, and dangerous-just like shorting the positive and negative together on a 6V battery,.

This is the same as paralleling 2 cells to balance them, except the voltages are much closer together. A cell charged to 3.3V and one charged to 3.4V will have 0.1V of difference between them. That is enough for some current flow, and enough for the cells to (in theory) balance. With Lead Acid it will mostly work. With LiFePo4 it will not, because the 2 cells will converge to 3.35V before the state of charge between them is the same.

That is why it is called ....top ..... balance. The voltage must be brought into the knee with the cells in parallel in order to parallel top balance.

 

[wholybee]

This is a couple of google results

There are two basic approaches to balancing: Passive balancing drains charge from cells having too much charge and dissipates drained energy as heat. Active balancing moves charge from “high cells” to “low cells,” attempting to conserve energy in the battery pack.

Passive balancing allows the stack to look like every cell has the same capacity as the weakest cell. Using a relatively low current, it drains a small amount of energy from high SoC cells during the charging cycle so that all cells charge to their maximum SoC. ... Passive cell balancer with bleed resistor.

There seems to be an implied belief that cells will balance without any such interaction. I believe that is a myth - which I am trying to either affirm or bust. I have found nothing in any video that explicitly says so one way or the other which maybe leading to a popular gross misunderstanding. I would like to see Will explicitly point that out as there seem to be some folks that expect that osmosis to work.

Both of the balancing approaches you describe involve cells connected in series.

 

[2Big2B]

Thank you. I appreciate your effort to explain it to me.

However, everyone seems to be missing the point. I am being treated like a small child asking how immaculate conception works. You can carefully and politely explain how human reproduction works until you are blue in the face but I will continue to probe with my simple question about how reproduction can take place without intercourse. It becomes a circular argument because that bible story isn't about REPRODUCTION - it is about fervent religious folklore. It is about magic. It is understood to be SUPERNATURAL. In that fable Christ was not a reproduction, but a supernatural new creation. Excuse me for this unfortunate analogy. I don't mean to drag religion into this or offend anyone. That isn't my intent. I am just getting frustrated by some apparent miscommunication.

The old saying is "You can't argue with ignorance".

I know all about the facts of life and don't need it explained to me. That isn't my point. I also know all about how electricity works, or would be dead many times over - and don't need that explained to me either.

Somehow my simple question about how something that seems to be supernatural has turned into a some weird dogmatic argument.

Last night I was chastised and "banned for two weeks" because I was not "interested in learning" but in arguing and being annoying - sort of like a small child who responds to each explanation with "but why? ... but why?? ... but why ???" I thought the spirit of this forum was to learn from each other and nurture the hobby.

I was reprimanded with an announced two week suspended membership by "Snoobler" without warning or recourse- immediately gagged for persisting to ask for clarification to my simple question. Nobody seems to understand what I am trying to say, regardless of how I try to explain what my question is. Look at how I write. I am more literate and grammatically diligent than most, am I not?

Perhaps it is because the question to even be a question is so ridiculous that it just goes over everyones' head. It is exposing some apparent common myth that seemingly is accepted as reality.

The myth is that by some kind of spontaneous regeneration balancing between cells can occur without a closed circuit. That myth is being propagated by the innocent omission of a detail when there is an instruction about top balancing. You get to see the setup, but not the actual process, and I think that detail is that there must be some kind of closed circuit, beit a BMS connected using passive balancing or else a power supply. Some of seem to believe that the stand alone construction of a parallel configuration will result in balancing - without a closed circuit of any kind between positive and negative. THAT IS ABSURD. I think that there seems to be some mysticism about LiFeP04 battery science that is driving the myth. Else, I am an idiot. Yet I have managed to live to be 68 years, have a four year Bachelor of Science degree (as well as British A Levels in the social sciences), am a licenced radio technician, a retired computer engineer, am widely regarded as intellectually gifted and very much a self made man. How can I have survived and have been successful all these years and have some black hole in something as basic and simple as this.

I am not alone in questioning this. "Boondock Saint" seemed to agree with me last night, and at that point it seems to have become personal to "Snoobler", the acting moderator who decided to ban me.

I give up.

You all need to work it out between yourselves. I am not welcome here apparently. Believe whatever mysticism of the immaculate ionic transfiguration you want to, if that is what this really is, if it suits you. Apparently my out of the box curiosity is not appreciated here.

 

[Bob B]

You seem to be mixing passive balancing with a BMS .... and top balancing by connecting cells in parallel.

These are 2 completely different things .... so which one is your concern?

When 2 cells are connected in parallel .... there IS a complete circuit .... thru the cells. The negative and positives will be connected and current will pass thru each cell to complete a circuit.
Have you seen internal resistance readings for the cells?

 

[John Frum]

Here is a very simple circuit.
Current flows from charger.pos through the cell to charger.neg
charger.pos->cell.pos
charger.neg<-cell.neg

here is another simple circuit
high_cell.pos->low_cell.pos
high_cell.neg<-low_cell.neg

Hope that helps.

 

[John Frum]

The above demonstrates that there will be current flow based on potential difference.
But leaving unbalanced cells in parallel is not an effective way to top balance cells.
State of charge and voltage are best correlated at the extremes.
Having said that please refer back to this https://diysolarforum.com/threads/passive-top-balancing-how-why-does-it-work.24645/post-290597

 

[wholybee]

Thank you. I appreciate your effort to explain it to me.

However, everyone seems to be missing the point. I am being treated like a small child asking how immaculate conception works. You can carefully and politely explain how human reproduction works until you are blue in the face but I will continue to probe with my simple question about how reproduction can take place without intercourse. It becomes a circular argument because that bible story isn't about REPRODUCTION - it is about fervent religious folklore. It is about magic. It is understood to be unnatural. In that story, Christ was not a reproduction, but a new creature, if you will pardon my analogy. I know all about the facts of life and don't need it explained to me. That isn't my point.

Somehow my simple question about how something that seems to be something supernatural has turned into a heated dogmatic argument.

Last night I was chastised and "banned for two weeks" because I was not "interested in learning" but in arguing and being annoying - sort of like a small child who responds to each explanation with "but why? ... but why?? ... but why ???" I thought the spirit of this forum was to learn from each other and nurture the hobby.

I was reprimanded with an announced two week suspended membership by "Snoobler" without warning or recourse- immediately gagged for persisting to ask for clarification to my simple question. Nobody seems to understand what I am trying to say, regardless of how I try to explain what my question is. Look at how I write. I am more literate and grammatically diligent than most, am I not?

Perhaps it is because the question to even be a question is so ridiculous that it just goes over everyones' head. It is exposing some apparent common myth that seemingly is accepted as reality.

The myth is that by some kind of spontaneous regeneration balancing between cells can occur without a closed circuit. That myth is being propagated by the innocent omission of a detail when there is an instruction about top balancing. You get to see the setup, but not the actual process, and I think that detail is that there must be some kind of closed circuit, beit a BMS connected using passive balancing or else a power supply. Some of seem to believe that the stand alone construction of a parallel configuration will result in balancing - without a closed circuit of any kind between positive and negative. THAT IS ABSURD. I think that there seems to be some mysticism about LiFeP04 battery science that is driving the myth. Else, I am an idiot. Yet I have managed to live to be 68 years, have a four year Bachelor of Science degree (as well as British A Levels in the social sciences), am a licenced radio technician, a retired computer engineer, am widely regarded as intellectually gifted and very much a self made man. How can I have survived and have been successful all these years and have some black hole in something as basic and simple as this.

I am not alone in questioning this. "Boondock Saint" seemed to agree with me last night, and at that point it seems to have become personal to "Snoobler", the acting moderator who decided to go all fascist about it.

I give up.

You all need to work it out between yourselves. I am not welcome here apparently. Believe whatever mysticism of the immaculate ionic transfiguration you want to, if that is what this really is, if it suits you. Apparently my out of the box curiosity is not appreciated here.

There is nothing for us to work out. Those of us that have studied electronics at a college level have both worked out the math and solved simple (and much more complex) circuits on paper, and then proven them in a lab. It is demonstrable fact. The refusal of some people to believe that does not make it fiction.

You are welcome to try the experiment I posted about a few posts up. It proves what we are saying. I encourage people to learn, and I am sorry you were offended by others here trying to teach you. But, as a student, you won't get a warm reception from telling teachers they are wrong.

When you state there must be a closed circuit, you are missing the point. 2 cells connected in parallel *IS* a closed circuit. Current flows, that current is measurable, can do work, and can balance a cell.

Peace out.

 

[2Big2B]

You seem to be mixing passive balancing with a BMS .... and top balancing by connecting cells in parallel.

These are 2 completely different things .... so which one is your concern?

When 2 cells are connected in parallel .... there IS a complete circuit .... thru the cells. The negative and positives will be connected and current will pass thru each cell to complete a circuit.
Have you seen internal resistance readings for the cells?

Again, Non-Sequitur.

The only "connection" shared by the cells is the fact that their common poles are glued together. I think the problem is conceptual. I think what you are saying is that by doing that the entire bank becomes one big communal cell that somehow shares its chemistry. If that is what you are telling me, then that is some dimension worth exploring that I must be unaware of. Do tell...

 

[upnorthandpersonal]

The myth is that by some kind of spontaneous regeneration balancing between cells can occur without a closed circuit. That myth is being propagated by the innocent omission of a detail when there is an instruction about top balancing.

Let's start fresh. The below image shows the equivalent circuit model of a Lithium Ion battery:


Source: https://www.researchgate.net/figure...del-of-the-lithium-ion-battery_fig1_276035953

I quote from the publication: "The equivalent circuit model is shown and consists of three parts: (1) the equivalent ohmic internal resistor R s ; (2) the resistor-capacitor (RC) parallel network C p // R p (where R p is the equivalent polarization resistance and C p is the equivalent polarization capacitance), which is used to simulate transient responses of the battery during charging-discharging transients; and (3) the OCV v oc (h(t)), which is a nonlinear function of SoC h(t). The equivalent circuit model considers the current as the model control input and the terminal voltage as the measured output."

Let's put two of these cells, at different state of charge (and let's assume a noticeably different terminal voltage), in parallel with one another. This forms closed circuit (agreed?) whereby an imbalance of voc means that one cell will have a Vb (terminal voltage) higher than the other will lead to a current to flow from the higher potential to the lower until the two terminal voltages equalize - essentially charge from the cell with the higher potential moves to the cell with the lower potential, increasing voc in the cell that was at lowest, and decreasing voc in the cell that was at highest potential.

The issue with LiFePO4 is that while theoretically there is a difference in voc at low and high state of charge, in practice this is not the case across the majority of the state of charge except in the knees - meaning either at low state of charge, or high state of charge. See diagram below:



While there is a small difference in voltage between say 40% and 60% state of charge, the delta between these is so small that if we were to take a cell at 40% SoC and 60% SoC and we were to put these in parallel, the resulting current flowing from the cell at higher potential to the one at lower potential is a) very small and b) because of that any imbalance in SoC (20% in this case) would take a very long time (impractically so) to balance out.

However, the same can not be said for cells at the 'upper knee' in the chart. where large deltas in voltage actually occur at very low deltas in SoC. This is where you balance with a BMS, this is where you top balance with a power supply in the mix.

In summary, putting LiFePO4 cells in parallel balances them, in some cases, but not at random states of charge somewhere on the 'flat part' of the curve. This is not the same for all chemistries: LTO for example has a curve that is not flat like this and there paralleling cells without additional power supply has more effect. Same is true for lead acid. For LiFePO4, you balance in the knees where you have a clear voltage indicating state of charge, and where relatively large voltage deltas correspond to small percentages of state of charge.